Background: the emergence of aggression in children living in Welfare Centers is a prevalent problem that requires effective interventions for treatment and prevention. Researches have been conducted on interventions to reduce aggression in children, but the type of research that examines the effectiveness of Play-Based Resilience Training on aggression has been neglected. Aims: The aim of this study was to investigate the effectiveness of Play-Based Resilience Training on aggression in children under the supervision of the Welfare Organization. Method: The method of the present study was quasi-experimental with a pretest-posttest design, and a control group. The statistical population included children aged 5-7 years under the supervision of the Tehran Welfare Organization in 2019. The sample consisted of 30 children who were selected by the convenient sampling method and randomly assigned to the study and control groups (15 individuals in each group). The research tools included the Preschooler Aggression Questionnaires of Vahedi et al. (2008), and the Child and Youth Resilience Measure of Ungar and Liebenberg (2009), and the Play-Based Resilience Training Package (Mohammadi Shemirani, 2019). For the children in the study group, the resilience training package was implemented in 14 sessions, and each session lasted for one hour. The data analysis was conducted by using the Multivariate Analysis of Covariance. Results: The results showed that Play-Based Resilience Training significantly reduced aggression and its components (verbal, physical, active, and total aggression) except for the component of relational aggression in the study group participants (p< 0/01). Conclusions: according to the findings of the present study, it can be concluded that the Play-Based Resilience Training Package can be used as an efficient intervention to reduce aggression in children